The present invention relates generally to a method of removing flux residue formed on electronic assembly surfaces and interfaces during high temperature soldering operations using rosin based flux for semiconductor device interconnections in electronic module assemblies. More particularly, the invention relates to a semi-aqueous solvent based method employing high boiling point, halogen-free, and non-aromatic organic solvent based cleaning compositions, in conjunction with water rinse, for removing the flux residue formed on the electronic assembly surfaces during semiconductor device joining to ceramic or organic chip carriers.
Electronic assembly processes in microelectronic fabrication typically employ solder interconnections using a lead-tin (Pb/Sn) alloy for electrically joining a semiconductor device to a chip carrier, such as a ceramic substrate or a printed circuit board (PCB). Recently there has also been a focus on Pb-free solder alloys. An example of solder interconnections is C4 (controlled-collapse-chip-connection) technology, also called flip-chip bonding, where a semiconductor chip is attached to a substrate. This involves connecting an array of solder bumps on the semiconductor chip to bonding pads on the substrate by heating the assembly to solder reflow temperature in the presence of rosin based flux to form a solder connection.
In multilayer ceramic (MLC) products, solder bumps on silicon devices are generally Pb/Sn alloys of various compositions which are deposited by evaporation or plate-up techniques. Alternatively, Pb-free solder alloys of the type Sn/Cu/Ag and Bi/Sn are being investigated by the industry to replace Pb based solder alloys.
Solder interconnection processes with high melting solder alloys typically utilize a rosin-based flux, for example Alpha 102-1500 rosin flux (Alpha Metals Flux 102-1500), which is applied on the array of semiconductor device solder bumps or on the solder wettable pads on the substrate. Due to its viscosity and tackiness the flux helps maintain the alignment of the solder bumps to the substrate bonding pads. The flux also provides an oxide-free native metal surface on the Pb/Sn solder bumps on the device side by complexing with the surface oxide layer and thereby exposing the native metal underneath for metal-to-metal contact with the bonding pads on the substrate side. This provides bond integrity and long term reliability of the solder connections.
After alignment, the semiconductor device/substrate assembly is subjected to solder reflow in a furnace under N2 or forming gas (5% H2 in N2) using a temperature profile with a peak temperature depending on the solder type and composition. For example, with 97Pb/3Sn alloy, a temperature profile with a peak temperature of approximately 350 to 365xc2x0 C. is used. The processes for flip-chip attachment to a multilayer ceramic chip carrier using solder bumps is disclosed in U.S. Pat. Nos. 3,401,126 and 3,429,040 (Miller et al.), the disclosures of which are incorporated by reference herein.
Rosin flux is also used for solder connections in the fabrication of ball grid array (BGA), ceramic ball grid array (CBGA), ceramic column grid array (CCGA), Surface Mount Technology (SMT) discretes, and hermetic seal band attachment to provide surface wettability of contacting surfaces during solder reflow.
The high temperature solder reflow conditions cause the rosin flux constituents to undergo a thermal transformation generating low molecular weight species which vaporize and are mostly removed in the process. However some of the reactive species, especially the higher molecular weight species, remain on the various surfaces. During the cooling cycle, after the solder reflow, the solder hardens forming solder connections between the semiconductor device and the substrate bonding pads. At the same time the thermally activated residual species from the flux decomposition undergoes cross-linking reactions which result in a resinous/carbonaceous by-product known as flux residue. This flux residue forms on the solder connections and on all other surfaces, including under the chip, on the semiconductor device and substrate that are exposed to the volatile species during solder reflow processing.
This cross-linked flux residue must be removed before subsequent operations can be performed. Failure to clean the flux residue can lead to reliability problems in long term use due to the possibility of stress corrosion when the assembly is exposed to a temperature and humidity environment. It is desirable to remove the flux residue prior to applying and curing an underfill material if such a material is used to encapsulate the solder connections for fatigue life enhancement and protection from the detrimental effect of environmental exposure. Failure to clean the flux residue can lead to voids in underfill coverage and adhesion failure resulting in device function reliability problems.
Rosin flux materials which are derived from the various Pine species are natural products comprising a complex mixture of cyclic hydrocarbon acids which constitute almost 90 percent of the rosin flux chemical composition along with a small fraction of polymerized rosin and about 10 percent of a neutral fraction constituting the corresponding esters, alcohols, acetate, and decarboxylated products. Referring to FIG. 1 there is illustrated rosin acid structures I-IV, which are the major components of the rosin flux. Abietic acid (I) is the predominant component along with dehydroabietic acid (II), dihydroabietic acid (III), and tetrahydroabietic acid (IV). The rosin flux is known to promote wetting of metal surfaces due to the complexation reaction of the rosin acids in the flux with the oxide layer on the solder surface. The rosin flux also provides an oxide-free exposed metal surface of high surface energy which thermodynamically should readily wet the contacting metal surfaces on the substrate and thereby provide reliable chip-to-substrate interconnection.
Commonly employed methods of cleaning the rosin flux residue left after high temperature solder reflow in the process of device to substrate interconnection involves the use of chlorinated solvents such as tetrachloroethylene and aromatic hydrocarbons such as xylene. More recently solvent compositions for flux residue cleaning consisting of trans-1,2-dichloroethylene (1,2-DCE), fluorochlorocarbons, hydrofluorocarbons (HFC) blends containing 1,2-DCE as the major component, hydrofluoroethers (HFEs), or mixture thereof, have become available as a replacement of tetrachloethylene in flux residue cleaning. These halogenated solvents, however, are undesirable due to associated environmental and disposal issues. This limits their desirability for use in industrial applications.
The use of xylene as a flux residue cleaning solvent also has concerns since it is a highly flammable volatile organic compound (VOC) with a flash point of about 85xc2x0 F., a boiling point of approximately 135-145xc2x0 C., a high vapor pressure, and a high evaporation rate. The use of xylene requires high cost explosion proof equipment and chemical safety measures in the manufacturing environment as well as regulatory compliance for air emissions in the case of VOC""s and hazardous air pollutants (HAPS).
There are a number of solutions proposed by others which provide alternate organic solvents that are relatively safe and mostly exempt from strict environmental regulations, as well as water-based cleaning solutions and the necessary equipment for alternate organic solvent and water-based cleaning.
Bolden et al., U.S. Pat. No. 5,340,407, the disclosure of which is incorporated by reference herein, describes a process of removing soldering flux and/or adhesive tape residue from a substrate. Bolden uses terpene-based cleaning compositions for flux residue removal from the surface of a printed circuit board, and also for the removal of the adhesive tape residue.
Bakos et al., U.S. Pat. No. 4,274,186, the disclosure of which is incorporated by reference herein, describes a cleaning composition containing N-methyl-2-pyrrolidone and an alkanolamine for flux residue cleaning from the surface of printed circuit boards.
Bengston et al., U.S. Pat. No. 5,431,739, the disclosure of which is incorporated by reference herein, describes environmentally safe flux removing compositions using aryl alcohols such as benzyl alcohol in water as a cleaning medium for solder flux residue from mildly activated rosin flux (RMA), oils and other contaminants from the surface of printed circuit boards.
Buchwald et al., U.S. Pat. No. 5,112,517, the disclosure of which is incorporated by reference herein, is concerned with using halogenated hydrocarbons, such as dichlorodifluoroethanes in conjunction with alkanols for removing rosin flux and flux residues from printed circuit boards.
None of the references listed above are concerned with removal of rosin flux residue formed on under-the-chip surfaces when rosin flux such as Alpha 120-1500 is used for high temperature solder interconnections. This process requires high temperature solder reflow at peak temperature above 300xc2x0 C. Examples of this high temperature solder reflow process include BGA, CBGA, and CCGA module assemblies. Accordingly, there exists a need for an alternative method of removing flux residue which is free from the problems associated with the traditionally used solvents referenced above.
Sachdev et al., U.S. Pat. No. 5,938,856, the disclosure of which is incorporated by reference herein, describes flux residue cleaning solvent compositions and a process for removing flux residue from under-the-chip where solder compositions used for device chip joining to ceramic substrate required peak solder reflow temperature from 320xc2x0 C. to about 365xc2x0 C. in the presence of high temperature rosin Alpha-102 flux. The solvent compositions described employ environmentally friendly non-halogenated and non-aromatic solvents for the residue cleaning cycle and isopropanol (IPA) or other low boiling solvents for the follow-on rinse cycle. Although this method is an improvement over the existing methods, the cleaning solvent compositions and the rinse solvents have a relatively low boiling point and flash point, a high evaporation rate and high vapor pressure which requires the use of costly safety equipment in a manufacturing environment.
Notwithstanding the prior art there remains a need for an improved method for the effective removal of flux residue from electronic components after solder interconnections, particularly under the device flux residue, formed during high temperature solder reflow conditions in the presence of rosin based flux compositions.
Accordingly, it is a purpose of the present invention to provide an improved method for the effective removal of flux residue from electronic components after solder reflow using high temperature solder alloys in the presence of rosin based flux compositions that is based on the use of environmentally friendly solvents with no chemical safety issues.
It is another purpose of the present invention to provide a non-hazardous semi-aqueous solvent based method of removing flux residues formed during solder reflow on the surfaces and under-the-chip in electronic component assembly fabrication processes.
It is another purpose of the present invention to provide an environmentally friendly method of cleaning flux residue that accumulates on the device chip and the chip carrier surface/interface regions during C4 interconnections or flip-chip bonding processes involving a high temperature solder reflow thermal profile.
It is another purpose of the present invention to provide an alternate method of cleaning flux residue which effectively eliminates the need for chlorinated and/or fluorinated solvents, and aromatic hydrocarbon solvents such as xylene.
It is another purpose of the present invention to provide an environmentally safe method of cleaning rosin flux residue from electronic assembly surfaces following solder joining process using evaporated or plated Pb/Sn solder, lead-free solder alloys, for example, Sn/Cu/Ag, Sn/Ag, Sn/Bi, or soft solder paste made with 80Au/20Sn alloy.
It is another purpose of the present invention to provide an improved method of cleaning post-chip join flux residue using a solvent cleaning composition constituting a high boiling hydrophobic solvent and an ionic and/or non-ionic surfactant, followed by water rinse to remove the cleaning solvent from the various exposed surfaces of the assembled electronic module.
It is another object of the present invention to provide an alternate method of cleaning flux residue employing organic solvents which have no major environmental regulatory health issues, no chemical safety concerns, and which have high boiling point and high flash point, low evaporation rate, and low vapor pressure.
These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.
The purposes and advantages of the present invention have been achieved by providing a method for removing rosin flux residue from assembly surfaces, interfaces and under device surfaces which comprise the steps of:
(a) providing a first cleaning composition which comprises a first water insoluble hydrophobic solvent with a surface active agent;
(b) immersing the assembly in the first cleaning composition and soaking the assembly in the first cleaning composition for 10 to 20 minutes at 50 to 90xc2x0 C. with intermittent agitation;
(c) removing the assembly from the first cleaning composition;
(d) immersing the assembly in a second cleaning composition which comprises a second water insoluble hydrophobic solvent and soaking the assembly in the second cleaning composition for 10 to 20 minutes at 50 to 90xc2x0 C. with agitation, preferably an immersion spray;
(e) removing the assembly from the second cleaning composition;
(f) applying a third cleaning composition which comprises a hydrophilic water soluble solvent, typically a propylene glycol methyl ether solvent, preferably applied by immersion with pressure spray, at approximately 50 to 75xc2x0 C. to the assembly for about 5 to 10 minutes;
(g) applying a water rinse, preferably a deionized water rinse, at approximately room temperature to 65xc2x0 C. to the assembly for approximately 5 to 10 minutes;
(h) blowing gas on the assembly and then heating the assembly to approximately 80 to 120xc2x0 C. to dry the assembly and thereby complete the removal of the flux residue from the assembly surfaces, interfaces and from under the device surfaces. The heating step at elevated temperature is preferably performed under vacuum.
The first water insoluble hydrophobic solvent and the second water insoluble hydrophobic solvent are propylene glycol alkylethers represented by the formula ROxe2x80x94(C3H6O)Nxe2x80x94C3H6OH wherein R is selected from the group consisting of propyl, butyl, pentyl and isobutyl and where N=0 to 4, and preferably 1 to 3. The surface active agent is an ionic surfactant or a combination of ionic and non-ionic surfactants.
The ionic surfactant or combination of ionic and non-ionic surfactants are preferably about 5 to 25 weight % of the first water insoluble hydrophobic solvent. The ionic surfactants are predominantly abietic acids selected from the group consisting of abietic acid, dihydrabietic acid, tetrahydroabietic acid, dehydroabietic acid, and mixtures thereof. Preferably Alpha 102-1500 rosin flux in benzyl alcohol. For example, approximately 60-70% (weight %) rosin flux and approximately 30-40% (weight %) benzyl alcohol. The second water insoluble hydrophobic solvent is preferably the same composition as the first water insoluble hydrophobic solvent.
The non-ionic surfactant is selected from the group consisting of rosin acid ester derivatives, abietyl alcohol, dihydroabietyl alcohol and mixtures thereof The non-ionic surfactant may also be an alkyl polyglycoside based surfactant, ethoxylated propoxylated aliphatic alcohol, or mixture thereof, preferably a low foam surfactant blend, Glucopon LF-1 (Cognis Corp., previously Henkel Corp.).
The hydrophilic water soluble propylene glycol methyl ether solvent is represented by the formula CH3Oxe2x80x94(C3H6O)Nxe2x80x94C3H6OH where N=0 to 4, and preferably 1 to 3.